1. Field of the Invention
The present invention relates to a cathode active material for a lithium ion secondary battery which includes a lithium manganese borate compound and a manganese oxide. The present invention also relates to a method for producing the cathode active material.
2. Description of the Related Art
In recent years, olivine phosphates LiMPO4 (M=Fe, Mn, Co, etc.) have been used as electrode active materials. The olivine phosphates are known to undergo less reduction in capacity even after a number of cycles due to their high stability. However, the olivine phosphates have theoretical capacities insufficient to meet applications where high capacity secondary batteries are needed.
Lithium borates LiMBO3 (M=Fe, Mn, Co, etc.) have been proposed as alternatives to olivine phosphates. The borate-based materials have received great attention as substitutes for lithium phosphates including (PO4)3− due to the presence of the triangle oxyanion (BO3)3−, which is lighter than other anions. For this reason, the borate-based materials are known to have higher theoretical capacities (approx. 220 mAh/g) than the phosphate-based materials. It is also known that the borate-based materials have the ability to increase the volume energy density of batteries due to their similar density to lithium phosphate. According to previous reports, however, resistances structurally occurring in batteries impede sufficient utilization of high theoretical capacities inherent to borate-based materials and permit only specific capacities as low as 80 mAh/g.
On the other hand, the use of manganese-containing compounds as cathode materials was proposed based on the fact that manganese (Mn) of LiMnBO3 generally has a higher oxidation-reduction potential than iron (Fe). From a theoretical point of view, the Mn-containing compound LiMnBO3 is known as a cathode material whose operating voltage is higher than that of the Fe-containing compound LiFeBO3. However, Mn-based borate materials suffer from limitations, such as lower capacity than LiFeBO3, owing to their inherent low electrical conductivity and ionic conductivity.